For tagDoubleBuiltIn where style is Auto , the last historized value is 4 whereas current value is 5.
For tagDoubleBuiltin_Discrete the last value in history is 5 which is same as current value.
Why is it that when style is Auto, the previous value is added to the history after next value change ?
This happens with Double / Float tags and not with Integer tag.
You're seeing the difference between discrete and analog settings (see the complete section below excerpt came from on this page in the manual for more details):
The Deadband and Analog Compression
The deadband value is used differently depending on whether the Tag is configured as a Discrete Tag or as an Analog Tag. Its use with discrete values is straight forward, registered a change any time the value moves +/- the specified amount from the last stored value. With Analog Tags, however, the deadband value is used more as a compression threshold, in an algorithm similar to that employed in other Historian packages. It is a modified version of the 'Sliding Window' algorithm. Its behavior may not be immediately clear, so the following images show the process in action, comparing a raw value trend to a "compressed" trend.
The Deadband Style property sets the: Auto, Analog, or Discrete.
When set to Auto, this setting will automatically pick from Analog or Discrete, based on the datatype of the Tag.
If the datatype of the Tag is set to a float or double, then Auto will use the Analog Style.
If the datatype of the Tag is any other type, then the Discrete style will be used.
The delayed value storage you're seeing is due to analog compression mode selected via Auto setting when tag is of type float or double. It's not actually storing the last value; instead it's storing the last value that fell within the deadband when a new value falls outside the deadband (follow link to manual above for a detailed explanation of how the analog deadband works).
